Point-contact rectifier



June 14, 1949.

4 AM WW4 I &. e 5. ILY mm "a f Filed March 1, 1945 F. H. BRITTAIN ET AL POINT-CONTACT RECTIFIER ATTORNEY Patented June 14, 1949 POINT-CONTACT RECTIFIER Francis Hugh Brittain, Pinner, and Charles Eric Ransley, Sudbury, England, assignors to The General Electric Company Limited, London,

England Application March 1, 1945, Serial No. 580,484 In Great Britain November 8, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires November 8, 1963 Claims. (Cl. 175-366) This invention relates to point contact rectifiers that is to saycombinations of a semi-conducting crystal (usually nowadays consisting mainly of silicon often with small additions of aluminium or beryllium or both) and a metal Wire (usually of tungsten) whose pointed end rests on the crystal. The pointed end may be shaped to give contact substantially at a point or along a line. The terms point contact or pointed end are intended to cover both forms. The invention relates also to their manufacture.

When such rectifiers are used at frequencies of the order of 10,000 mc./s., the capacitance of the contact is of great importance; it should be as small as possible. The object of this invention is to reduce this capacitance.

We have found that an increase of the hardness of the pointed end of the wire may lead to a decrease of the smallest attainable capacitance, and that the hardness of the usual tungsten point may be conveniently increased for this. purpose by carbonising it in known manner. The reason for this decrease in capacitance is due to a lessening of the deformation of the tungsten point, the hardening of the tungsten point rendering it more resistant to such deformation.

According to one aspect of the invention, the

manufacture of a point contact rectifier comprises the step of subjecting the said pointed end to a hardening treatment;

According to another aspect of the invention, in a point contact rectifier the said wire is of tungsten and its pointed end is carbonised. The single figure of the drawing illustrates a suitable construction for a point contact rectifier of the type herein described.

The wire usually has a bent portion that provides the elasticity necessary in order that the pressure of the point on the crystal may be regulated. This part is preferably not hardened. Again when the point has been hardened it is liable to penetrate the surface layer of the crystal on 'whose presence eflicient rectification depends. The tendency to penetrate may be decreased by subjecting the point to electrolytic etching of known kind; the effect of the etching is probably to smooth the surface of the point.

In one method of manufacture according to the invention, the wire is of tungsten 0.2 mm. in diameter with the end sheared off at an angle of about 45 to the axis of the wire to form a pointed end. The necessary elasticity is provided in known manner, by an S-bend a short distance from the pointed end. The pointed end is carbonised and the bent portion is excluded from such, carbonising by bringing the pointed end into contact for a few seconds with a carbon block heated to 1500-1600 C. in a non-oxidising atmosphere. The pointed end is then etched for a few seconds by the use of alternating current and an alkaline solution.

In another method of manufacture according to the invention, the wire is again of tungsten, its pointed end is shaped, and the wire is provided with an S-bend, as in the previous example. The local carbonising of the pointed end, whilst excluding the bent portion from such carbonising, is achieved by introducing the wire into a bell jar in which is a stout filament of suitable material such as tungsten, heated to a temperature of 1500-2000" C. The said pointed end is brought to touch the stout tungsten wire for a few seconds whilst an atmosphere containing hydrocarbon vapour is passed through the .bell jar. A suitable atmosphere is nitrogen, 20% hydrogen, saturated with xylene vapour. The saturation may be produced by bubbling the nitrogen and hydrogen mixturethrough the hydrocarbon at room temperature. The pointed "end is then etched as in' the previous example.

A' suitable co-operating'cry'stal is one of the kind described in Un'ited'States Letters Patent No. 2,419,561, granted April 29, 1947, to Jones et al. The rectifier may be completed in any known or suitable manner by mounting the point contact in such a manner that its pointed end bears with a suitable pressure upon a surface of the crystal. For example, and referring to the single figure of the drawing, a point contact rectifier .may conveniently be constructed by the use of a hollow cylindrical ceramic body member l0 having conductive terminals ll, l2 threaded into individual ends thereof. The pointed metal wire l3 includes an unhardened S-bend as heretofore mentioned and'is soldered at its blunt end into a coaxial bore ll of the terminal II. The semi-conducting crystal I5 is soldered on'the end of a conductive pin l6 which is adjustably positioned in an axial bore ll of the terminal I I and is retainedin adjusted position therein by a set screw l8 threaded through the side of the terminal l I. During assembly of the rectifier, the pin.

justed position. The ceramic body 10 is provided with an aperture I! by which the cavity within the body may conveniently be filled with a wax to maintain the metal wire I! in adjusted position on the semi-conducting crystal l5 and also to render the point contact rectifier moisture proof.

It has been found possible with the aid of the present invention to achieve readily capacities of the order of 0.05-0.10 micro-microfarads. With previously known methods the capacity is 0.25- 0.5 micro-microfarads or even greater.

We claim:

1. A method of manufacturing a point contact rectifier comprising a semi-conducting crystal and a metal wire whose pointed end rests upon said crystal which comprises the step of subjecting said pointed end to a hardening treatment.

2. A method according to claim 1, wherein said metal wire has a bent portion to provide the elasticity necessary in order that the pressure of the point on the crystal may be regulated, and wherein said bent portion is excluded from said hardening treatment.

3. A point contact rectifier comprising a semiconducting crystal and a metal wire whose pointed end rests upon said crystal, wherein said metal wire is of tungsten and the pointed end thereof is carbonised.

4. A point contact rectifier comprising a semiconducting crystal and a metal wire whose point end rests upon said crystal, wherein said metal wire has a bent portion to provide the elasticity necessary in order that the pressure of the point on the crystal may be regulated and wherein the pointed end of said wire is harder than said bent portion.

5. A point contact rectifier according to claim 3, wherein said hardened. pointed end has an etched surface.

6. A method of manufacturing a point-contact rectifier comprising a semi-conducting crystal and a tungsten wire having a pointed end resting upon said crystal which comprises the step of hardening said pointed end of said tungsten wire by subjecting said pointed end to a carbonising treatment.

7. A method of manuiacturing a point-contact rectiiier comprising a semi-conducting crystal and a tungsten wire having a pointed end resting upon said crystal which comprises the step of hardening said pointed end of said tungsten wire by bringing said pointed end into contact with carbon heated to a suitable high temperature in a non-oxidising atmosphere.

8. A method of manufacturing a point-contact rectifier comprising a semi-conducting crystal and a tungsten wire having a pointed end resting upon said crystal which comprises the step of hardening said pointed end of said tungsten wire by heating said pointed end to a suitable temperature in an atmosphere containing a hydrocarbon vapour.

9. A method of manufacturing a point-contact rectifier comprising a semi-conducting crystal and a tungsten wire having a pointed end resting upon said crystal which comprises the step of hardening said pointed end of said tungsten wire by heating said pointed end to a suitable high temperature in an atmosphere containing a hydrocarbon vapour, said heating of said pointed end being effected by bringing said pointed end into contact with a heated tungsten wire.

l0. A method of manufacturing a point-contact rectifier comprising a semi-conducting crystal and a metal wire having a pointed end resting upon said crystal which comprises the steps of subjecting said pointed end to a hardening treatment, and subsequently subjecting said hardened pointed end to electrolytic etching in order to reduce the tendency of said hardened pointed end to penetrate the surface layer of said crystal.

FRANCIS HUGH BRITTAIN. CHARLES ERIC RANSLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,611,653 Lilianfeld Dec. 21, 1926 1,708,572 Hartmann et a1. Apr. 9, 1929 

